So-called “hybrid modalities”, such as PET-CT, SPECT-CT, PET-MR and SPECT-MR for example, have become increasingly important in medical imaging in recent times. These refer to the following:
PET: Positron Emission Tomography
CT: Computed Tomography
SPECT: Single Photon Emission Computed Tomography
MR: Magnetic Resonance Tomography
The advantage of such combinations is the connection of one modality with high local resolution (in particular MR or CT) to a modality with high sensitivity (in particular SPECT or PET). Reference is made below to a combined PET-MR system. Embodiments of the present invention can however generally be used for all forms of hybrid modalities or associated measuring methods.
An MR system generally includes a control computer, a measuring system and an image computer. A measuring program runs on the control computer, activating the measuring system to record MR images according to the planned measuring sequence. A measuring program here generally includes a number of program steps and optionally measuring breaks, during which operators can for example adjust the patient support or administer contrast agent to a patient. Each program step is assigned a measuring protocol, which controls the physical parameters of the measuring system for measuring purposes.
PET uses the particular characteristics of the positron emitter and positron annihilation to determine the functions of organs or cell regions quantitatively. For this corresponding radiopharmaceuticals marked with radionuclides are administered to the patient before the examination. As they decay the radionuclides emit positrons, which after a short distance interact with an electron, with the result that so-called annihilation occurs. This produces two gamma quanta, which fly apart in opposing directions (with a 180° offset). The gamma quanta are captured by two opposing PET detector modules within a specific time window (coincidence measurement), with the result that the annihilation site is determined at a position on the connecting line between these two detector modules.
To show this the PET detector module must generally cover a large part of the gantry arm length. It is divided into detector elements with side lengths of a few millimeters. During detection of a gamma quantum each detector element generates an event record, indicating the time and evidence site, i.e. the corresponding detector element. This information is transmitted to a high-speed logic unit and compared there. If two events coincide at a temporal maximum interval, a gamma decay process on the connecting line between the two associated detector elements is assumed. The PET image is reconstructed using a tomography algorithm, i.e. so-called back projection.
The different requirements for the modalities combined in a hybrid modality in respect of measurement planning means that the planning of an optimum measuring sequence is a demanding task for operators. For optimum diagnostic evaluation of data records acquired using hybrid modalities it is essential to prepare for and carry out the examination in an appropriate manner.
Established methods use sequential recording by both modalities. This means that a sequential order results even during acquisition planning. CT measurements and PET measurements are thus planned one after the other.
For simultaneous acquisition by two modalities, as is possible for example with the PET-MR hybrid modality, it is however desirable to have an option which both enhances user-friendliness and also optimizes the quality of the result data.
MR examinations deploy techniques, with which a fairly large region of the body can be examined, in that the patient couch with the patient supported thereon is passed through the magnet, with examinations being carried out in different couch positions. This makes it possible to examine regions of the body which are larger than the examination volume available to the system.
A body region is examined here, which is larger than the available image field, in that a number of so-called levels are measured. The body region is broken down into individual segments and a measurement, which may for example contain a number of sub-measurements, is taken for each segment in an associated couch position. By examining different body regions it is possible for example to record the body as a whole, with this being done at different levels (couch positions or imaging regions). The images recorded at each level each have measuring parameters such as echo time, repetition time, layer thickness, number of layers, voxel size, layer orientation, etc. for example.